The present invention pertains generally to pulsed sounder imaging and more specifically to high resolution, three dimensional imaging of a large number of independently moving targets.
There are two broad classes of methods for generating pictures of target arrays, i.e., optical methods and raster scanning methods. Optical imaging produces a discernible picture by receiving light signals from the entire target array simultaneously, and employs a lens to sort, or focus, the various signals to form the image. Three dimensional pictures are achieved by using either stereographic viewing systems or by employing holographic techniques. A time series of images must be viewed to obtain motion information of targets in an array using optical imaging. The closest optical analogue to the present invention is the imaging Michelson Interferometer as disclosed by G. G. Shepherd, W. A. Gault, R. A. Koehler, J. C. McConnell, K. V. Paulson, E. J. Llewellyn, C. D. Anger, L. L. Cogger, J. W. Haslett, D. R. Moorcrott and R. L. Gettinger, "Optical Doppler Imaging of the Aurora Borealis", Geophys. Res. Ltrs. of Vol. II No. 10, pp. 1003 to 1006 (1984).
Fourier transform holography techniques have been used in an attempt to image electromagnetic radiation outside of the visible spectral region, but such techniques require extensive sensor arrays.
The other method of generating images of a target array comprises raster scanning which produces a picture by scanning, or mapping, the target array. Raster scanning requires that a narrow sensor beam be formed to obtain spatial resolution. The object being imaged must then be scanned at a rate which is faster than any significant changes in movement of the object (target). Radial velocities of objects (targets) can be measured using the raster scanning system by determining the doppler frequency, but this occurs only at considerable expense in resolution.
One area of particular interest for over thirty years in locating and imaging targets had been the use of medium frequency radars to explore the middle atmosphere, as disclosed by F. F. Gardner and J. L. Pawsey, "Study of the Ionospheric D-region Using Partial Reflections," J. Atmos. Terr. Phys. 3, 321-344, 1953. Since that time, there have been a considerable number of suggestions regarding the physical source of observed weak scattering. These include single target scattering from horizontally continuous layers as disclosed by Gardner and Pawsey, supra.; J. B. Gregory, "Radio Wave Reflections From the Mesosphere," J. Geophys. Res. 66, 429-445, 1961; volume scattering from small (as compared to a fresnel zone) turbulent irregularities in the electron density as disclosed by Gardner and Pawsey, supra.,; J. S. Belrose and M. J. Burke, "Study of the Lower Ionosphere Using Partial Reflection, 1. Experimental Technique and Method of Analysis," J. Geophys. Res 69, 2799-2818, 1964; W. A. Flood, "Revised Theory for Partial Reflection D-region Measurements," J. Geophys. Res. 73 (17), 5585-5598, 1968; A. H. Manson, M. W. J. Merry and R. A. Vincent, "Relationship Between the Partial Reflection of Radio Waves From the Lower Ionosphere and Irregularities as Measured by Rocket Probes," Radio Sci. 4(10), 955-958, 1969; H. A. Von Biel, "Amplitude Distributions of D-region Partial Reflections," J. Geophys. Res. 76, 8365-8367, 1971; B. Tanenbaum, J. Samuel, H. Shapiro, and J. E. Reed, "Phase-difference Distributions in a D-region Partial Reflection Experiment," Radio Sci. 8, 437-448, 1973; D. B. Newman, Jr. and A. J. Ferraro, "Sensitivity Study of the Partial Reflection Experiment," J. Geophys. Res. 78, 774-777, 1973; volume scattering from irregularities in the electron neutral collision frequency as disclosed by W. R. Piggott and E. V. Thrane, "The Effect of Irregularities in Collision Frequency on the Amplitude of Weak Partial Reflections," J. Atmos. Terr. Phys. 28, 311-314, 1966; multiple large discrete irregularities as disclosed by J. B. Gregory and A. H. Manson, "Seasonal Variations of Electron Densities Below 100 km at Mid-latitude. I. Differential Absorption Measurements," J. Atmos. Terr. Phys. 31, 683-701, 1969; G. L. Austin and A. H. Manson, "On the Nature of the Irregularities That Produce Partial Reflections of Radio Waves From the Lower Ionosphere (70-100 km)," Radio Science 4(1), 35-40, 1969; G. L. Austin, R. G. T. Bennett, and M. R. Thorpe, "The Phase of Waves Partially Reflected From the Lower Ionosphere (70-120 km)," J. Atmos. Terrest. Phys. 31, 1299-1106, 1969; D. J. Cohen and A. J. Ferraro, Modeling the D-region Partial Reflection Experiment, Radio Sci. 8, 459-465, 1973; constructive interference from fortunately spaced vertical stratifications as disclosed by T. Beer, "D-region Parameters From the Extraordinary component of Partial Reflections," Ann. Geophys. 28, 341-347, 1972; a large number of small scatters, highly dispersed horizontally but highly concentrated vertically as disclosed by J. D. Mathews, J. H. Shapiro, and B. S. Tanenbaum, "Evidence for Distributed Scattering in D-region Partial-reflection Processes," J. Geophys. Res. 78(34), 8266-8275, 1973; irregularities caused directly by gravity waves as disclosed by C. O. Hines, "Internal Atmospheric Gravity Waves at Ionospheric Heights," Can. J. Phys. 38, 1441-1481, 1960; A. H. Manson, J. B. Gregory, and D. G. Stephenson, "Winds and Wave Motions (70-100 km) as Measured By a Partial-reflection Radiowave System," J. Atmos. Terrest. Phys. 35, 2055-2067, 1973; reflections from the top and bottom of a turbulent sheer flow layer as disclosed by K. Schlegel, A. Brekke, and A. Haug, "Some Characteristics of the Quiet Polar D-region and Mesosphere Obtained With the Partial Reflection Method," J. Atmos. Terrest. Phys. 40, 205-213, 1978; P. K. Rastogi and O. Holt, "On Detecting Reflections in Presence of Scattering From Amplitude Statistics With Application to D-region Partial-reflections, Radio Sci. 16(6), 1431-1443, 1981; oscillations caused by surface, or ducted waves as disclosed by R. M. Harper and R. F. Woodman, "Preliminary Multiheight Radar Observations of Waves and Winds in the Mesosphere Over Jicamarca," J. Atmos. Terrest. Phys. 39, 959-963, 1977; interference modes of acoustic waves as disclosed by K. Schlegel, E. V. Thrane, and A. Brekke, "Partial Reflection Results in the Auroral D-region Explained in Terms of Acoustic Waves," J. Atmos Terrest. Phys. 42, 809-814, 1980; and Bragg scattering from gravity waves as disclosed by R. M. Jones and R. N. Grubb, "D-region Partial Reflection Doppler Measurements With the NOAA/MPE Digital HF Radar, Max-Planck Institut Fur Aeronomie," Rpt. No. MPAE-W-02-80-20, September 1980; R. M. Jones, G. W. Adams, and D. C. Walden, "Preliminary Partial Reflection Measurements at Brighton, Colo. on 9 Jan. 1981, NOAA Tech. Memo," ERL SEL-80, 1982. These references are specifically incorporated herein by reference for all that they disclose.
There has been considerable development in the understanding of the nature of the scattering processess. Several studies have shown that there appear to be periodic wave patterns, rather than random patterns moving across antenna fields from E-region reflections, as disclosed by R. F. Kelleher, "Some Statistical Properties of the Ground Diffraction Patterns of Vertically Reflected Radio Waves," J. Atmos. Terrest. Phys. 28, 213-223, 1966; M. G. Golley and D. E. Rossiter, "Some Tests of Methods of Analysis of Ionospheric Drift Records Using an Array of 89 Aerials," J. Atmos. Terrest. Phys. 33, 701-714, 1971; W. Pfister, "The Wave-Like Nature of Inhomogeneities in the E-region," J. Atmos. Terrest. Phys. 33, 999-1025, 1971; and from D-region partial reflections as disclosed by M. G. Golley and D. E. Rossiter, "Some Tests of Methods of Analysis of Ionospheric Drift Records Using an Array of 9 Aerials," J. Atmos. Terrest. Phys. 3, 701-714, 1971. The specularity, or aspect sensitivity, of the scatters at medium frequencies was first noted by Gardner and Pawsey, supra. and has been studied by B. C. Lindner, "The Nature of D-region Scattering of Vertical Incidence Radiowaves I. Generalized Statistical Theory of Diversity Effects Between Spaced Receiving Antennas," Aust. J. Phys. 28, 163-170, 1975a; B. C. Lindner, "The Nature of D-Region Scattering of Vertical Incidence Radiowaves II. Experimental Observations Using Spaced Antenna Reception," Aust. J. Phys. 28, 171-184, 1975b; R. A. Vincent and J. S. Belrose, "The Angular Distribution of Radio Waves Partially Reflected From the Lower Ionosphere," J. Atmos. Terrest. Phys. 40, 35-47, 1978; K. L. Jones, "Angular Variation of Partial Reflections from the D-region Using a Steerable Beam Radar," J. Atmos. Terrest. Phys. 42, 569-575, 1979; W. K. Hocking, "Angular and Temporal Characteristics of Partial Reflections from the D-region of the Ionosphere," J. Geophys. Res. 84(A-3), 845-851, 1979; W. K. Hocking, "Investigations of the Movement and Structure of D-region Ionospheric Irregularities," Ph.D. Thesis, Physics Dept. Univ. Adelaide, Australia, 1981. The picture that has emerged from this work is that the scattering is highly specular below 75 kilometers, and most of the radar pulse is returned within a few degrees of vertical. Off-vertical scattering increases rapidly with increasing altitude, exceeding a ten degree width by 85 kilometers. Above the mesopause, the specularity has been observed to increase again. Measurements at VHF (P. Czechowsky, R. Ruster, and G. Schmidt, "Variations of Mesospheric Structures in Different Seasons," Geophys. Res. Lett. 6(6), 459-462, 1979; W. L. Ecklund and B. B. Balsley, "Long-term Observations of the Arctic Mesosphere with the MST Radar at Poker Flat, Alaska," J. Geophys. Res. 86, 7775-7780, 1981; B. B. Balsley, "The MST Technique--A Brief Review," J. Atmos. Terrest. Phys. 43(516), 495-509, 1981; J. Rottger, "Investigations of Lower and Middle Atmosphere Dynamics with Spaced Antenna Drifts Radars," J. Atmos. Terrest. Phys. 43(4) 277-292, 1981), have revealed considerable detail about the mesospheric scattering structures observed at 50 MHz. A high degree of specularity is observed at these frequencies also, except at high latitudes. The VHF results suggest a two-component scattering mechanism; one specular and sporadic, and the other isotropic and more constant in time. All of these references are specifically incorporated herein by reference for all that they disclose.
The structures that scatter medium frequency radar pulses appear to occur on scales smaller than can be resolved with a practical antenna beam so that there is no practical way of imaging the targets. For example, beam forming techniques produce a beam which is tens of kilometers wide at altitudes of 80 kilometers. Clearly, this size beam is incapable of providing sufficient resolution to image targets. Of course, the same problems exist in ultrasonic imaging and sonar imaging.
Apart from imaging, time domain interferometry techniques have been used by radio-astronomers for many years to locate distant targets such as distant stars and galaxies with a high degree of precision. Interferometry techniques are capable of determining the zenith angle to locate one specific target with high precision. The detected red shift of the target indicates its doppler frequency.
Consequently, time domain interferometry techniques have been used by radio astronomers to provide high resolution information pertaining to the zenith angle of a single target. This clearly overcomes the low-resolution problems associated with steered beams.
Pfister, supra., originally suggested that Fourier transformations could be performed on data reflected from targets and the phase differences compared to locate the target. This technique was attempted by D. C. Cox, N. Cainos, and A. T. Watermann, "A Technique for Obtaining the Doppler Spectrum from Sampled Amplitude-Phase Data in a Data-gathering Array," IEEE Trans. Ant. Prop. AP-18(4) 580.582, 1970, which utilized a physically steered beam and examined the Fourier transform of the returned data. Since Cox et al. utilized a physically steered beam, the resolution of the data was limited by the resolution of the physically steered beam.
The technique suggested by Pfister, supra. was later used by D. T. Farley, H. Ierkic, and B. G. Fejer for "Radar Interferometry: A New Technique for Studying Plasma Turbulence in the Ionosphere," J. Geophys. Res. 86, 1467-1472, 1981. As disclosed by Farley et al., two complex voltage signals were detected from each of two antennas from which both amplitude and phase data were derived. Interferometry techniques were then utilized by Farley et al. to locate the target by zenith angle in one dimension. Farley et al. then distinguished targets from noise by observing the data in a series of time interval returns to determine the temporal persistence of a potential target. If a potential target did not persist for several time intervals, it was determined to be noise. A three-antenna array formed in a triangle was later suggested by H. M. Ierkic and J. Rottger in "Mesopheric Measurements of Irregularity Patches Using a Three Antenna Interferometer", Proceedings, Second Workshop on Technical Aspects of MST Radar, Urbana, Ill. May 21-25, 1984. Ierkic et al. again utilized temporal persistence to distinguish targets from noise.
U.S. Pat. No. 4,172,255 issued Oct. 23, 1979 to Barrick et al. discloses an HF coastal current mapping radar system which utilizes two separate radar transmitters and receivers which look at specific points in the ocean to determine the radial velocity vector of the movement of surface currents of the ocean as a result of moving waveforms. The doppler frequency is used at each separate antenna location to determine the speed of movement of the waves. The radial velocities determined in this manner are combined trigonometrically to determine the true direction and speed of the waveforms. This provides information regarding movement of surface currents. As disclosed in FIG. 5 of the Barrick et al. patent, a particular point in the ocean is selected for investigation using the phase difference of the returned signals. This is the same technique used by Farley et al., supra., except that rather than determining a one-dimensional zenith angle, Barrick et al. determines a one-dimensional azimuth angle.
Consequently, Barrick et al. essentially performs the same techniques as disclosed by Farley et al. with the exception that a point is being selected in the azimuthal plane by Barrick et al., rather than a zenith angle, as disclosed by Farley et al. Other differences also exist between the manner in which the data is utilized. Farley et al. used the one-dimensional zenith angle data to distinguish targets from noise by recognizing the time persistence of the target over several sampling periods. The temporal persistence of the target functions to distinguish the target from noise. Barrick et al., on the other hand, is not interested in locating specified targets, but rather, locates specific points on the surface of the ocean using interferometry techniques. The selected specified point is then analyzed to determine the radial speed of the target relative to each of the two antenna locations. This inforamtion is analyzed trigonometrically to produce a vector velocity indicating the speed and direction of the wave motion.
Again, neither of these references uses a comparison of phase angles of the received data to eliminate noise. Barrick et al. merely compares phase values to select a specified point to analyze. Consequently, Barrick et al. has not even addressed the question of identifying the location of a detected target, but rather, uses interferometry techniques to select a predetermined location to be examined. Farley et al., on the other hand, relies upon temporal persistence of the target, as disclosed above, to distinguish the targets from noise. Both of these techniques rely upon data in a single plane, i.e., a zenith angle in a single plane, as disclosed by Farley et al. and an azimuth angle, as disclosed by Barrick et al.
Consequently, the prior art has failed to provide a device which is capable of generating three dimensional data with high resolution so that an image can be formed of an object. Moreover, prior art techniques have been unable to accurately distinguish noise from target data without losing temporal resolution.